1. Field of the Invention
The present invention concerns an Earth acquisition method and device for a three-axis stabilized satellite in Earth orbit at a low inclination (less than 10.degree.) to the Equator in order to return the satellite from a Sun pointing safeguard or standby attitude to its nominal Earth pointing attitude.
2. Description of the Prior Art
The three axes with respect to which a satellite is to be stabilized in a nominal attitude are an axis directed towards the Earth called the yaw axis usually denoted Z, an axis perpendicular to the plane of the orbit called the pitch axis denoted Y and an X axis completing the orthogonal axis system and called the roll axis.
Any three-axis stabilized telecommunication or observation satellite (these are typical missions in low inclination orbits) has at least one 2-axis Earth sensor and two 1-axis Sun sensors with orthogonal axes (or a single 2-axis Sun sensor) with fields of view substantially separate from that of the Earth sensor for detecting attitude errors and rate gyros for measuring angular speeds about the three axes.
Torque for changing attitude and/or angular speed is usually produced by actuators such as wheels (momentum or reaction wheels) and/or thrusters.
This set of sensors and actuators associated with one or more electronic measurement processors and torque calculators constitutes what is usually referred to as the orbit and attitude control system (OACS) of the satellite.
In the event of a serious fault in the OACS in the nominal orbit or in response to an erroneous telecontrol signal transmitted from the control station on the ground, the satellite may depart from its nominal attitude and take up a Sun pointing safeguard attitude with the X axis pointing towards the Sun (the 1-axis solar sensors are usually placed on this X axis) with the Y and Z axes rotating about the X axis at a constant speed, typically 0.5.degree./s. In the meantime, if necessary, an angular momentum generator system (wheels) will have been operated to reduce to zero any angular momentum of the satellite that is non-null on average in order to facilitate rotating the satellite about the Sun pointing direction. In this mode, the Sun is always within view of the Sun sensors but the Earth is usually not in view of the Earth sensor.
The Sun pointing attitude may be a standby nominal attitude, applying for example while the satellite is on a transfer orbit en route to its final station.
When the fault has been processed and corrected, either on board the satellite or from the ground, or at the time when the standby attitude is to be terminated, it is therefore necessary for the satellite to (re)acquire the Earth in order to begin (or resume) normal operation of the mission. It is standard practice (especially on EUTELSAT-II) to seek the Earth by rotation about an axis aligned with the direction of the Sun relative to a reference marker which is continuously measured on board the satellite by means of the Sun sensors until the Earth enters the field of view of the Earth sensor. The rotation is then stopped and Earth capture initiated. There are periods during the day when such rotation cannot be used, in particular between 20:00 and 04:00 satellite solar time, during which period the Sun is usually not in view of the 2-axis Sun sensors needed to control the rotation axis about which occurs the conical movement enabling the Z axis to cross the Earth. In this case a more general method using a star sensor, for example--in addition to the Sun and Earth sensors--is needed if the Earth is to be acquired even when the Earth/satellite/Sun geometry is not favorable to this.
Satellite attitude control concepts using a star sensor in addition to Sun and Earth sensors are already known and are the subject matter of, among others, French Patent No. 2,522,614 the Centre National d'Etudes Spatiales (inventors MOUILHARAT, DUCHON, GUILBERT and ROLFO) entitled "Configuration de satellite a orbite equatoriale a moyens solaires perfectionnes" ("Equitorial orbit satellite configuration with improved solar means"); French Patent No. 2,637,565 to AEROSPATIALE (inventor MAUTE) entitled "Systeme de controle actif selon trois axes d'un satellite geostationnaire" ("Active three-axis control system for a geostationary satellite"); and European Patent No. 0,338,687 to BRITISH AEROSPACE (inventor MATTHEWS) entitled "Method of and apparatus for returning an Earth orbiting spacecraft to an Earth pointing attitude after displacement therefrom".
In the first patent there is no disclosure as to acquisition of the Earth from the Sun pointing attitude and the satellite embodies a platform and a payload rotatable relative to the platform between which the sensors are disposed.
The second patent suggests for the change from the Sun pointing attitude to the nominal attitude the principle of scanning a series of stars by a star sensor as the satellite rotates about its roll axis in an attitude such that the roll axis remains parallel to the projection of the direction of the Sun in the plane of the Equator, followed by rotation in pitch about the direction defined by the Pole Star to reacquire the Earth by means of the Earth sensor. This document is extremely vague as to how this principle is to be put into practice, however, in particular with regard to detection of the direction representing the projection onto the Equatorial plane of the direction of the Sun and then as to identification of the Pole Star.
The third patent is slightly more precise as to how the Earth is reacquired. It proposes a satellite provided with a 2-axis Earth sensor, a 2-axis Sun sensor and a star sensor mounted on the satellite so as to be able to sense the star CANOPUS in the Southern hemisphere; this satellite must be provided with an angular momentum generator. The reacquisition process starts conventionally by rotating the satellite with the Sun sensor pointing at the Sun (this is the conventional Sun pointing attitude). When CANOPUS enters the field of view of the star sensors, rotation is stopped and an angular momentum is generated by activating the generator so as to confer gyroscopic stiffness about a fixed direction of the satellite. Rotation about the Sun direction continues by an angle calculated using the date and astronomical tables so that subsequent rotation about the instantaneous direction of the angular momentum axis will bring the Earth into the field of view of the Earth sensor.
This disclosure presupposes the use of a star sensor with a very wide field of view as CANOPUS is more than 35.degree. away from geographical South; it is in any event doubtful that in any configuration simple rotation in Sun pointing mode would necessarily bring CANOPUS into the field of view of the star sensor, unless it is assumed that this rotation could take place over a long period (several hours) which would be incompatible with most missions. Also, nothing is stated as to how CANOPUS is identified. Finally, the necessity to rotate the satellite about the Sun direction after generation of the angular momentum (a priori transverse to the rotation axis) introduces parasitic phenomena which are difficult to control because of the gyroscopic stiffness.
An object of the invention is to alleviate the inadequacies and drawbacks mentioned above by proposing a method and device enabling Earth acquisition from a Sun pointing attitude mode in any Earth/satellite/Sun configuration within a timespan compatible with modern mission requirements (typically less than 30 minutes) by fast and reliable intermediate sensing of the Pole Star (which is less than 1.degree. away from geographical North).
Of course, to do this the satellite must have a star sensor having a sufficiently wide field of view (greater than or equal to 2.times.(orbital inclination+1.degree.)) and capable of sensing the Pole Star (visual magnitude=2.3). Also, this sensor must be disposed on the satellite so that its optical axis is preferably parallel to or near the (-Y) axis of the satellite.